This invention relates to instrument transformers and more particularly to an electrostatic shield for controlling the electrostatic field in a split core instrument transformer.
Instrument transformers include current transformers and voltage transformers and are used to measure the properties of electricity flowing through conductors. Current and voltage transformers are used in measurement and protective applications, together with equipment, such as meters and relays. Such transformers “step down” the current and/or voltage of a system to a standardized value that can be handled by associated equipment. For example, a current transformer may step down current in a range of 10 to 2,500 amps to a current in a range of 1 to 5 amps, while a voltage transformer may step down voltage in a range of 12,000 to 40,000 volts to a voltage in a range of 100 to 120 volts. Current and voltage transformers may be used to measure current and voltage, respectively, in an elongated high voltage conductor, such as an overhead power line.
A conventional current transformer for measuring current in a high voltage conductor typically has a unitary body with an opening through which the conductor extends. Such a conventional current transformer has a unitary core, which is circular or toroidal in shape and has a central opening that coincides, at least partially, with the opening in the body. With such a construction, the current transformer is mounted to the conductor by cutting and then splicing the conductor. As can be appreciated such cutting and splicing is undesirable. Accordingly, current transformers having two-piece or split cores have been proposed. Examples of current transformers having split cores are shown in U.S. Pat. No. 4,048,605 to McCollum, U.S. Pat. No. 4,709,339 to Fernandes and US20060279910 to Gunn et al.
The control of electrostatic field stress is an issue in a split core current transformer having a high voltage conductor disposed between the split core segments, one of which core segments has a low voltage conductor wound thereon. Uncontrolled electrostatic field stress between the high and low voltage conductors can cause partial discharges that will eventually erode the insulating material between the high and low voltage conductors and the split core segments. While electrostatic shields are available to reduce the electrostatic field stress experienced between high and low voltage conductors, there is room for improvement in electrostatic shields.
Accordingly, the present invention is directed to an electrostatic shield for controlling the electrostatic field in a current transformer.